Meltblowing and spunbond processes are commonly employed to manufacture nonwoven webs and laminates. With meltblowing, a molten thermoplastic is extruded from a die tip to form a row of filaments or fibers. Converging sheets or jets of hot air impinge upon the fibers as they are extruded from the die tip to stretch or draw the fibers, thereby reducing the diameter of the fibers. The fibers are then deposited in a random manner onto a moving collector belt to form a nonwoven web.
With spunbond processes, continuous fibers are extruded through a spinneret. Air is directed at the extruded fibers to separate and orient them. The fibers are collected onto a moving collector belt. At a downstream location, the fibers are consolidated by passing the layer of fibers through compacting roller, for instance. The spunbond process frequently utilizes quenching air to cool the extruded before they contact the collector belt.
Large volumes of air are used during both the meltblown and spunbond process. Moreover, much of the air is heated and moving at very high velocities, sometimes approaching the speed of sound. Without properly collecting and disposing of the process air, the air would likely disturb personnel working around the manufacturing apparatus and other nearby equipment. Further, the heated air would likely heat the surrounding area in which the nonwoven is being produced. Consequently, attention must be paid to collecting and disposing of this process air.
Managing the process air is also important to producing a homogeneous nonwoven web across the width of the web. The homogeniety of the final nonwoven web depends greatly on the air flow around the fibers as they are deposited onto the collector belt. For instance, if the air flow velocity is not uniform in the cross-machine direction, the fibers will not be deposited onto the collector belt uniformly, yielding a non-homogeneous nonwoven web.
Various air management systems have been used to collect and dispose of the process air. One particular air management system uses a collecting duct situated below a perforated collector belt to collect and dispose of the process air. An air moving device, such as a fan or vacuum pump, is connected to the collecting duct to actively draw the air into the collecting duct. The collecting duct is comprised of a plurality of a smaller air passageways arranged side-by-side in a rectangular grid. The grid includes a central row of air passageways extending across the machine width and upstream and downstream air passageways flanking either side of the central row. The central row of air passageways is disposed directly below the extrusion die in what is commonly referred to as the forming zone. Each air passageway includes an inlet and an outlet with a 90 degree elbow in between. An air moving device is operatively connected to each outlet to draw the process air into the individual inlets.
As mentioned above, the air flow velocity of the process air around the collector belt should be uniform, especially in the machine direction at the forming zone, to form a homogeneous nonwoven web. Achieving a uniform air flow velocity, however, has proven challenging. In the collecting duct described above, moveable dampers are associated with each outlet of the air passageways. To achieve uniform air flow velocity with this collecting duct, an technician must manually manipulate each damper until the air flow velocity is sufficiently uniform. In some instances, the technician may be unable to achieve a uniform air flow velocity no matter how much time and effort is spent adjusting the dampers. Moreover, the dampers must be readjusted each time a different fiber material or process air flow rate is used. Thus, the operator must readjust the dampers virtually every time the process is started or an operating condition is changed. The readjustment process takes a great deal of time and may ultimately yield a nonuniform air flow velocity regardless of how the moveable dampers are adjusted.
What is needed, therefore, is an air management system that can collect and dispose of the process air so as to produce a uniform air flow velocity at the collector belt, especially around the forming zone. The air management system should be designed such that dampers and other manual controls are not necessary, even over a wide range of process air flow rates.